Method for manufacture of a lead acid storage battery

ABSTRACT

The present invention relates to a lead acid storage battery and provides a lead acid storage battery having a high durability against complicated vibrations such as perpendicular and horizontal vibrations and the combination of these vibrations applied by vehicles or machines on which the battery is installed and having a long shelf life. A polyolefin resin having good fluidity is heat molten and poured into two portions above and near the left and right ends of the plate group comprising positive and negative plates having lugs at nearly the center portion so that the resin reaches at least one of the inner wall or inner partition wall, thereby to form a beam-shaped plate fixing member.

This is a division of application Ser. No. 08/441,129 filed May 15,1995.

BACKGROUND OF THE INVENTION

The present invention relates to lead acid storage batteries. Morespecifically, the present invention is concerned with a lead acidstorage battery excellent in vibration resistance of plate groups and amethod for making the same.

PRIOR ART

As for lead acid storage batteries used in automobiles, althoughrecently the vibration applied to the batteries while driving ofautomobiles has diminished with the increase in the paving rate of road,the mechanical strength of the strap for connecting plate groupsdecreases owing to corrosion caused by an increase of temperature ordecrease in the amount of electrolyte resulting in breakage of thebatteries. Furthermore, lead acid storage batteries used in agriculturalmachinery and construction machinery receive a great vibration appliedby the machinery and are required to have a high mechanical strength oftheir constructive parts.

In order to meet the demand, it has been proposed to make a lead acidstorage battery having vibration resistance by injecting and filling anadhesive comprising an epoxy resin in the form of a strip in one or twoportions above the plate group contained in each of the cell chambersprovided in the case to fix the plate group at the upper portionthereof, thereby inhibiting the shaking of the plate caused byvibration.

On the other hand, as materials for the case of lead acid storagebatteries used in automobiles, there are generally used polypropyleneresins which are light in weight and excellent in chemical resistanceand small in raw material cost.

These polypropylene resins have no or very low adhesion to other resinsunless they are subjected to any treatment and hardly adhere to theepoxy resins filled in the portion above the plate group to fix theplate group.

As a result, the epoxy resin filled in the portion above the plate groupacts effectively to fix together the positive plate and the negativeplate which constitute the plate group or fix integrally the plate groupcontaining the positive and negative plates and envelope separators, butcannot be integrated with the inner wall of the cell chamber of the casecontaining the plate group. Therefore, when a strong vibration isapplied to lead acid storage batteries so fabricated, positionaldeflection between the plates constituting the plate group can beprevented by the fixing action of the epoxy resin, but the plate groupwhich is heavy in weight is apt to shake in the cell chamber of thecase, causing concentration of a force on the strap connecting the lugof the plates of the same polarity or on the connector between theadjacent cells. Thus, breakage or degradation of these portions isbrought about.

Accordingly, the substantial vibration resistance is still insufficient.

SUMMARY OF THE INVENTION

A principal object of the present invention is to solve the aboveproblems and to provide lead acid storage batteries excellent invibration resistance even under the condition of strong vibration.

Another object of the present invention is to provide a method formaking lead acid storage batteries superior in workability and excellentin vibration resistance.

The lead acid storage batteries which attain the above objects haveplate groups comprising a plurality of positive plates alternating witha plurality of negative plates and envelope separators between theplates, the envelope separators extending to a position higher than theupper end of the plates, each of the plates having a lug and the lugs ofthe plates of the same polarity being integrally connected by aconnecting strap; a case comprising a thermoplastic synthetic resin inwhich a plurality of cell chambers containing the plate groups areformed by partitioning the inside of the case by inner partition walls;and a cover which covers an opening of the case, the upper end of theplate group contained in the cell chamber of the case being integrallyfixed by a beam-shaped insulating synthetic resin provided above theplate group, at least one of left and right ends of the beam-shapedsynthetic resin being fixed to the inner wall of the case or thepartition walls by fusion bonding.

The thermoplastic synthetic resin which forms the case is suitablypolypropylene because of its light weight, excellent chemical resistanceand low raw material cost as mentioned above and the materials of thebeam provided above the plate group are preferably polyolefins includingpolyethylene and polypropylene. The method for making the lead acidstorage battery according to the present invention comprises forming aplate group by alternately arranging a plurality of positive plates, aplurality of negative plates and a plurality of envelope separatorsextending above the plates and integrally connecting lugs of the platesof the same polarity by a connecting strap, putting a plate group ineach of a plurality of cell chambers produced in a case made of athermoplastic synthetic resin by partitioning the inside of the case byinner partition walls, pouring an electrically insulating resin heatedto molten state into the portion above the plate group so that themolten resin crosses the plate group in the direction of its thicknessand both the left and right ends of the thus formed beam of molten resincontacts with inner wall of the case or the inner partition wall andthen curing the poured molten resin by cooling to form a beam-shapedplate fixing member. The heat molten resin is poured in the followingmanner. That is, two shape regulating plates having a length which isthe same as or somewhat narrower than the width (in the depth direction)of the cell chamber of the case, namely, the interval between the innerpartition walls, are inserted into the portion above the plate group ineach cell chamber at a right angle to the arranging direction of theplates and the molten resin is poured into the space between the twoshape regulating plates and is cured by cooling.

The two shape regulating plates serve as a guide which controlsunlimited effluence of the poured molten resin and thereby forming abeam-shaped plate fixing member. Therefore, the interval between the twoopposing plates determines the width of the plate fixing member.Furthermore, the molten resin poured into the space between the tworegulating plates is preferably a polyolefin resin such as polyethylene,polypropylene or the like considering the fusion bonding to thethermoplastic synthetic resin such as polypropylene which forms thecase. The poured molten resin contacts with the inner wall of the caseor the inner partition wall to give heat to the wall portion to melt atleast the surface of that portion and fusion bonds to the wall portionduring the period of cooling to cure and thus forming a beam-shapedplate fixing member. The molten synthetic resin is poured into the spacebetween the two shape regulating plates from an injection nozzle and thenozzle moves in the space at a speed nearly the same as the speed of themolten resin which flows down from the upper end of the envelopeseparator to the upper end of the plate.

The polyolefin resin molten made by heating preferably has a fluidity ofat least 30 in terms of melt index value, and preferably this resin ispoured into the portion above the plate group and solidified in suchstate as embracing the upper part of the plates and thus, a beam-shapedplate fixing member can be surely formed.

The two regulating plates contact with the molten resin and hold theshape at the time of curing by cooling, and it is preferred to treat thesurface of the regulating plates contacting with the resin with amaterial having both the releasing effect and heat resistance, forexample, fluorocarbon polymers such as polytetrafluoroethylene in orderto smoothly carry out removal of the plates after curing of the resin,namely, smoothly carry out so-called releasing.

By employing such construction, the upper parts of the positive andnegative plates and the envelope separators of the plate group containedin each cell chamber are fixed as a whole by the beam-shaped platefixing member made of a synthetic resin or the upper parts of theenvelope separators are caught into or fusion bonded to the fixingmember and thus, integrally fixed by the fixing member, and furthermore,at least one end of the left and right ends of the beam-like platefixing member is fusion bonded to and integrated with the side wall ofthe case or the inner partition wall which forms the cell chamber.Accordingly, even if vibration is applied to the battery, the plategroup is prevented from moving or deflecting in the cell chamber.Especially since one end of the beam-shaped plate fixing member isintegrated with the side wall of the case or the inner partition wall,the ability to fix the position of the plate group is enhanced and theforce of vibration is inhibited from being concentrated on theconnecting strap which integrates the lugs of the plates of the samepolarity and on the cell connector which is provided linking to theabove-mentioned connecting strap and is connected, through the innerpartition wall, to a connecting strap of the plates of differentpolarity of the plate group contained in the adjacent cell chamber. As aresult, falling off of active material caused by positional deflectionof plates, fatigue of metal due to the concentration of vibration, andcracking or breaking of the upper part of the plates, the connectingstraps or the cell connectors caused by the fatigue of metal can beprevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional oblique view of the lead acid storage battery madein Example 1 of the present invention.

FIG. 2 shows the state of bonding between the plate group and the platefixing member in the lead acid storage battery made in Example 1.

FIG. 3 is a graph which shows the relation between melt index value ofthe resin which forms the plate fixing member and vibration resistanceof the battery.

FIG. 4 is a sectional view of another embodiment of the plate fixingmember.

FIG. 5 is a sectional oblique view of the lead acid storage battery madein Example 2 of the present invention.

FIG. 6 explains the shape of the plate fixing member made in Example 2.

FIG. 7 is a sectional view of the plate fixing member formed in Example2.

Other characteristics and advantages of the present invention will beexplained in detail in the following examples.

EXAMPLE 1

FIG. 1 illustrates the lead acid storage battery in Example 1 of thepresent invention and especially shows the state of fixing of plategroup in the cell chamber in the case.

This lead acid storage battery has an output voltage of 12 V and acapacity of 30 Ah and the size of the container comprising the case andthe cover is 200 mm in height, 233 mm in length and 120 mm in width.Length (in depth direction) of each of the six cell chambers formed inthe lengthwise direction (in the lining direction of the cell chamber)of the case is 37 mm. Plate group 1 of about 114 mm in height, about 100mm in width and 37 mm in total thickness is put in pressurized state ineach cell chamber.

The plate group 1 comprises, for example, five positive plates 2, sixnegative plates 3 and envelope separators 4 between adjacent positiveplate and negative plate which envelope each of the positive plate 2,these positive plates, negative plates and envelope separators beingalternately arranged. The envelope separators 4 are prepared by cuttinga microporous polyolefin resin sheet, for example, a microporouspolyethylene sheet to a given size, folding the sheet in two in the formof U and integrally fixing both the left and right ends of the foldedsheet. The upper end of the folded sheet extends to a position higherthan the upper end of the positive plate enveloped in the folded sheet.

Lugs 5 of the positive and negative plates are provided so that those ofthe same polarity are situated in a straight line and near the centralpart above the plate group and they are electrically and mechanicallyintegrated by the connecting straps 6 for positive plates and negativeplates. Cell connector 7 is provided integrally at one end of theconnecting strap 6 by welding. This cell connector 7 is welded to aconnecting strap for the plates of different polarity in the adjacentcell chamber through inner partition wall 9 provided for forming cellchambers in the case 8.

Thus, the six cells are all electrically connected in series to maintainan output voltage of 12 V. FIG. 1 shows a cutaway view of the leftmostfirst cell chamber of the six cell chambers. Therefore, a connectingpole which connects to a terminal for all the negative plates in thebattery is provided at the connecting strap for negative plates in thiscell in place of a cell connector. For the same reason, although notshown, a connecting pole which connects to a terminal for all thepositive plates in the battery is provided at the connecting strap forpositive plates contained in the last sixth cell chamber.

Furthermore, as shown in FIG. 1, since the lugs of the positive andnegative plates and the connecting straps which connect and integratethe lugs are provided near the central part above the plate group, theleft and right end sides above the plate group have spaces greater thanthose in conventional general batteries.

A polyolefin resin, polyethylene here, kept in molten state by heatingto about 200° C. and having a fluidity of at least 30 in terms of meltindex value was poured in the form of a streak into the portions nearthe left and right end above the plate group by an injection nozzle toform beam-shaped plate fixing member 10.

The melt index here is an index which shows the fluidity ofthermoplastic resins such as a polyolefin resin at plasticized state.This is also called melt flow index or melt flow rate and is a widelyused index which is specified in flow test method for thermoplastics ofJIS K7210.

Since the polyethylene which forms the beam-shaped plate fixing member10 has a high fluidity of 30 or more in melt index value in a heated andmolten state, not only does it reach the envelope separators 4 higher inheight than the plates, but also it enters between the envelopeseparators to reach the upper portions of the positive plates 2 andnegative plates 3, and it catches the upper end of the envelopeseparators 4 thereinto to give heat to the envelope separators resultingin partial fusion of the envelope separators. As a result, thepolyethylene is cooled in such a state to fix the upper parts of theplates.

Therefore, the positive plates, negative plates and envelope separatorsare fixed and integrated at two portions, namely, by the two beam-shapedplate fixing members in addition to the connecting strap 6 uponsolidification or curing of the polyethylene by cooling.

Thus, since the plate group is fixed and integrated by the twoconnecting straps 6 and the two beam-shaped plate fixing members,namely, at four portions in total and in addition, since the plate groupper se is put in pressurized state in the cell chamber, deflection inposition or movement between the plates and between the plate and theenvelope separator caused by vibration can be sufficiently prevented.

Furthermore, since polypropylene is used as a material of the case andpolyethylene which is a polyolefin resin is used as a material of theplate fixing member 10, even when the heated and molten polyethylene ispoured after inserting the plate group in the molded and sufficientlyhardened case, a part of the heat possessed by the molten polyethyleneper se is given to the inner wall of the case or the surface of theinner partition wall which contacts with the polyethylene to soften theportion of the inner wall or the surface of the inner partition wallresulting in fusion bonding between the inner wall of the case or theinner partition wall and the plate fixing member. Accordingly, the plategroup is also integrated with the case through the beam-shaped platefixing member and hence, breakage of the connecting straps or theconnectors due to concentration of vibration or stress thereon does notoccur and a construction having markedly high vibration resistance isprovided.

FIG. 2 is a partial sectional view which shows the bonding state betweenthe plate fixing member 10 and the plate group 1 and between the member10 and the leftmost inner wall of case 8 or the inner partition wall 9.

FIG. 3 is a graph which shows the relation between melt index value ofthe plate fixing member 10 and vibration resistance of a batteryprovided with this plate fixing member 10. When the melt index value ofthe plate fixing member 10 is 30 or more, the battery generally shows ahigh vibration resistance.

On the other hand, the case is made of a polypropylene having a meltindex value of 9 and a melting point of about 160° C., and the moltenpolyethylene poured keeps a temperature of about 200° C. and a meltindex value of 30 or more, and these are both polyolefin resins.Therefore, they fusion bond to each other and the bonding strength ishigh. The plate fixing member 10 here may be any of polymer resinsmainly composed of olefins in addition to polyethylene and they showrelatively high fusion bonding strength.

When the case is made of normally used crystalline polypropylene and thebeam-shaped plate fixing member 10 is also made of polypropylene, thepolypropylene of the plate fixing member 10 is preferably amorphous. Theamorphous polypropylene in a heated and molten state gives heat to apart of the crystalline polypropylene which forms the case to melt thepart and satisfactorily adapt thereto to provide a high fusion bondingstrength.

Therefore, as the materials of the plate fixing member 10, there may beused not only polyethylene and polypropylene, but also any otherpolyolefin resins as far as they can be fusion bonded.

Since the above-mentioned polypropylene case has an melt index value of9, moldability and shock resistance required for cases are sufficientlymaintained.

In pouring the heated and molten polyethylene or polypropylene into thecell chamber in the case, when a nozzle is positioned opposing nearlythe center of the thickness of the plate group in the vicinity of leftand right portions above the plate group, the poured molten resin justunder the nozzle is most strongly pushed by the pouring pressure(gravity of the resin per se) and then begins to flow in both the leftand right directions or along the envelope separators in the form of astreak. This results in covering a wide area of the upper surface of theplate group. However, it is preferred to restrict the area of the uppersurface of the plate group to be covered with the resin to at most 80%in order to smoothly carry out pouring of electrolyte in fabrication oflead acid storage battery and discharging of gas generated with chargingof the battery into the gas phase portion in the upper part of the cellchamber. The minimum covering area must be about 2% though it depends onthe height of the beam-shaped plate fixing member 10 formed upon curingof the poured resin by cooling.

Irrespective of the beam-shaped fixing member being maximum one orminimum one, as shown in FIG. 4 of longitudinal section along thelengthwise direction of the beam-shaped plate fixing member 10, theportion of the formed member just under the nozzle is thicker than otherportions. As a result, the strength to fix the plate group at thethicker central portion is higher than the portions in the vicinity ofthe left and right ends.

EXAMPLE 2

The beam-shaped plate fixing member can be provided in the form having asection longer in lengthwise direction, namely, a rectangular section,using shape regulating plates as a pouring frame for the molten resin.An example thereof is shown in FIG. 5.

In FIG. 5, 11 indicates a beam-shaped plate fixing member made ofpolyethylene and having a rectangular section and having a size of 37 mmin length which is the same as the width of the cell chamber, 13 mm inthickness and about 20 mm in height. Both the left and right ends in thelengthwise direction fusion bond to the inner side wall of the cell andthe inner partition wall.

By increasing the height of the beam-shaped plate fixing member 11having a rectangular section, the strength of fixing and integrating theupper part of the plate group and the strength of fusion bending to andintegrating with the side wall of cell chamber and the inner partitionwall increase and as a result, the strength of fixing the plate group inthe cell chamber can be increased and vibration resistance can beimproved.

The method of forming the beam-shaped plate fixing member having arectangular section will be explained referring to FIG. 6.

In FIG. 6, 12 indicates shape regulating plates. A pair of these plateswhich are opposed to each other are fitted to base 13 to form a shapefixing jig. In this case, width X of the shape regulating plate (guideplate) 12 is the same as or somewhat narrower than the interval betweenthe inner partition walls 9, namely, the length in depth direction ofthe cell chamber, the distance Y between the opposing two guide platesis the thickness of the fixing member to be formed, and the height Z isthe distance from the upper end of the case to the upper end of theenvelope separator 4, namely, to nearly the upper surface of the plategroup. The numeral 14 is a hole for inserting a nozzle provided at thebase 13 and 15 indicates a nozzle for pouring a molten resin, forexample, polyethylene.

The jig shown in FIG. 6 is for forming one plate fixing member andcomprises base 13 to which a pair of shape regulating plates 12 arefixed. Such jig is employed because poles as output and input terminalsare provided at the first cell and the last sixth cell in the case andhence, it is convenient to form the plate fixing members separately.However, for the second to fifth cells which have no pole, two pairs ofopposing shape regulating plates are provided at a base having a lengthcorresponding to the length (in depth direction) of the cell chamber andtwo pairs of the guide plates can be arranged being opposed to theportions near the left and right end above the plate group,respectively, and a molten resin can be poured from a nozzle insertedthrough the hole for inserting the nozzle to form the plate fixingmembers.

The specific method for forming the plate fixing member is as follows.The shape fixing jig is fitted to the desired cell in the case andnozzle 15 is inserted from the nozzle inserting hole 14 of the shapefixing jig and a resin of relatively high viscosity (MFR=50 g/10 min) inthe molten state, for example, polyethylene, is discharged therefrom.The polyethylene in the molten state discharged from the nozzle flowsdown in the vertical direction of the plate by its own fluidity andflows between the envelope separators extending to the position higherthan the plate and simultaneously spreads in the planar directioncrossing the upper surface of the plate group to contact with the innerpartition wall and furthermore, it tries to spread in the direction ofarrangement of the plates, but the planar spread of the polyethylene inthe direction of arrangement of the plates is hindered by the two shaperegulating plates 12 and the polyethylene stays in the rectangular spaceformed by a pair of the opposing two shape regulating plates 12 and theinner partition wall 9. Thereafter, the molten resin flows by gravity inthe direction of the upright plates (the resin can flow only in thisdirection) and reaches the upper frame of the positive and negativeplates which is positioned lower than the upper end of the envelopeseparators. When the molten resin is cured in this state, as shown inFIG. 7, the upper frame of the positive and negative plates and envelopeseparators 4 are fixed to form an integral plate group and furthermore,the beam-shaped insulating plate fixing member 11, the both ends ofwhich are fixed to the inner wall of the case or the inner partitionwall 9, is formed.

In this construction, the interval Y between the opposing shaperegulating plates 12 is preferably set at 2-80% of the width of theplate as aforementioned. If the interval Y is less than 2% of the widthof the plate, the thickness of the beam-shaped plate fixing member 11formed is too thin and the strength is insufficient and sufficientfixing effect cannot be obtained. If the interval Y is more than 80% ofthe width of the plate, the resulting beam-shaped plate fixing memberconsiderably hinders the diffusion of the poured electrolyte to causedecrease of capacity of the battery. For satisfying the fixing strengthof plates and plate group and pouring operation of electrolyte, theproportion of the width of the plate fixing member 11 in the width ofthe plate is preferably about 10-30%.

Furthermore, by subjecting the inner surface of the opposing shaperegulating plates with which the molten resin contacts to surfacetreatment such as fluorine treatment for the purpose of providingreleasing effect, the shape regulating jig can be easily removed whencuring of the molten resin is completed or the surface of the moltenresin reaches curing state and as a result, workability for forming theplate fixing members can be improved.

Usually, when a battery receives a vibration, each of the plates isgoing to vibrate or vibrates around a pivot which is the lug 5 fixed bythe strap and as for the plate group, this is going to vibrate orvibrates around a pivot which is the connecting strap or the cellconnector. However, according to the construction as formed by themethod of the present invention, vibration of the plates and the plategroup in the cell chamber can be sufficiently inhibited by the effect offixing the plates and the plate group by the two beam-shaped platefixing members.

As explained above, according to the present invention, a plate groupcomprising a plurality of positive plates alternating with a pluralityof negative plates and envelope separators between adjacent plates, saidenvelope separators extending to a position higher than the plates, thelugs of the plates of the same polarity being connected by a strap and aconnector, is put in each of the cell chambers formed in the case, andan insulating resin in the molten state is poured into the portionsabove the plate group in such a manner that both ends of the resincontact with the inner wall of the case or the inner partition wall,thereby to form a beam-shaped plate fixing member. This plate fixingmember fixes the upper portions of all plates and envelope separators toinhibit positional deflection of the plates and vibration of the plategroup in the cell chamber when vibration is applied to the battery.Thus, lead acid storage batteries of high reliability are provided.

What is claimed is:
 1. A method for making a lead acid storage batterywhich comprises the steps of:(a) forming a plurality of plate groups,with each plate group being formed by alternately arranging a pluralityof positive plates and a plurality of negative plates, each plate havinglower and upper ends and a lug disposed on the upper end, envelopingeach of the positive plates or each of the negative plates in anenvelope separator having an upper portion extending to a positionhigher than the upper end of the plate, and integrally connecting all ofthe positive plates together with a first connecting strap andintegrally connecting all of the negative plates together with a secondconnecting strap; (b) placing the plate groups in a plurality of cellchambers formed in a case by a plurality of inner partition walls, thecase being made of a synthetic thermoplastic resin; (c) inserting twoshape regulating plates having a width the same as or narrower than aninterval between opposing inner partition walls into an area above theplate group in one of the cell chambers, the shape regulating platesbeing disposed in the area above the plate group at a right angle to thedirection of the thickness of the plate group and forming a spacebetween the shape regulating plates; (d) pouring a heat molteninsulating resin into the space between the shape regulating plates toform a beam of resin with first and second ends and crossing the plategroup in the direction of the thickness of the plate group, with thefirst end of the beam contacting an inner wall of the case or one of theinner partition walls, and the second end of the beam contacting one ofthe inner partition walls; and (e) curing the molten resin by cooling toform a beam-shaped plate fixing member.
 2. A method according to claim1, wherein the space between the two shape regulating plates is set at2-80% of the width of the plate group.
 3. A method according to claim 2,wherein the space between the two shape regulating plates is set at10-30% of the width of the plate group.
 4. A method according to claim1, wherein the shape regulating plates are subjected to a surfacetreatment to provide a releasing effect on the resin.
 5. A methodaccording to claim 1, wherein the case is made of polypropylene and theheat molten insulating resin is a polyolefin resin.
 6. A methodaccording to claim 5, wherein the heat molten resin is polyethylene. 7.A method according to claim 5, wherein the heat molten polyolefin resinhas a fluidity of 30 or more in terms of melt index value.
 8. A methodaccording to claim 1, wherein the heat molten resin is poured into thespace between the shape regulating plates from an injection nozzle.
 9. Amethod according to claim 8, wherein molten resin flows down from theupper portion of the envelope separator while the nozzle moves in thespace at a speed substantially the same as the speed at which the moltenresin flows down from the upper portion of the envelope separator.
 10. Amethod according to claim 1, wherein the envelope separator is made froma microporous polyolefin sheet to provide for fusion bonding between atleast part of the upper portion of each of the envelope separators tothe plate fixing member.